The invention relates to a process which uses peroxy esters as polymerization initiators for preparing crosslinked bead-form polymers having high swellability and low contents of soluble fractions.
Ion exchangers having very uniform particle size (termed "monodisperse" below) have gained in importance in very recent times, since economic advantages can be achieved in many applications as a result of the more favourable hydrodynamic properties of an exchanger bed made from monodisperse ion exchangers. Monodisperse ion exchangers can be obtained by functionalizing monodisperse bead polymers.
One of the ways of preparing monodisperse bead polymers is the process known as seed/feed, in which a monodisperse polymer ("seed") is swollen in the monomer and this is then polymerized. Seed/feed processes are described, for example, in European Patent Specifications 98 130(U.S. Pat. No. 4,419,245) and 101 943(U.S. Pat. No. 4,564,644).
It is said that seed polymers should have a high swelling index, so that they can absorb a large amount of the monomer added during the seed/feed process. The swelling index (SwI) is the quotient obtained from the volume of the swollen polymer and the volume of the unswollen polymer. The swelling index can be controlled in a known manner through the content of crosslinker: low contents of crosslinker give high swelling indices and vice versa. Thus, for example, styrene polymers which are crosslinked using from 0.2 to 0.5% by weight of divinylbenzene have swelling indices of from 5 to 10 in toluene. However, seed polymers which have a low degree of crosslinking have a very high fraction of soluble polymers which are not crosslinked. This fraction of soluble polymers, which are not crosslinked, in the seed polymer is undesirable from many points of view:
1. The polymerization of the swollen seed may be impaired due to agglomerations of the particles with one another caused by polymer fractions dissolved out from the seed by the added monomer. PA0 2. The functionalization for preparing the ion exchangers may be made more difficult because the polymer fractions which are dissolved out build up in the reaction solution used for functionalization. PA0 3. The end products (ion exchangers) may contain increased amounts of soluble polymers, and this can lead to undesirable leaching of the ion exchangers. PA0 tert-butyl peroxyacetate, PA0 tert-butyl peroxyisobutyrate, PA0 tert-butyl peroxypivalate, PA0 tert-butyl peroxyoctoate, PA0 tert-butyl peroxy-2-ethylhexanoate, PA0 tert-butyl peroxyneodecanoate, PA0 tert-amyl peroxyneodecanoate, PA0 tert-amyl peroxypivalate, PA0 tert-amyl peroxyoctoate, PA0 tert-amylperoxy-2-ethylhexanoate and PA0 tert-amyl peroxyneodecanoate. PA0 2,5-bis(2-ethylhexanoylperoxy)-2,5-dimethylhexane, PA0 2, 5-dipivaloyl-2,5-dimethylhexane and 2,5-bis(2-neodecanoylperoxy)-2,5-dimethylhexane. PA0 di-tert-butyl peroxyazelate and PA0 Di-tert-amyl peroxyazelate.
The object of the present invention is to provide crosslinked bead-form polymers having high swellability and low contents of soluble fractions.